1. Field of the Invention
The present invention relates to an electric tool battery provided with a plurality of cells connected in series and configured to be capable of multiple monitoring of the respective voltages of the individual cells by a plurality of voltage monitoring elements at the time of charging and discharging.
2. Description of the Related Art
As shown in FIG. 6, in an electric tool battery 100, in order to prevent overcharge of cells S1 through Sn, the voltages of the individual cells S1 through Sn are doubly monitored by two voltage monitoring elements 101 and 102. Thus, if one voltage monitoring element is out of order, the other voltage monitoring element operates to prevent overcharge or the like. In FIG. 6, the voltage monitoring element for monitoring the cells other than the cell S1, i.e., the cells S2 through Sn, is omitted.
FIG. 7 is a schematic perspective view of a specific wiring structure for guiding the connection portion between the negative electrode of the cell S1 and the positive electrode of the cell S2 to the two voltage monitoring elements 101 and 102. The negative electrode of the cell S1 and the positive electrode of the cell S2 are electrically connected to each other by a lead plate 104, and a voltage signal extraction portion 104v in the form of a band plate is formed at the center of the upper side of the lead plate 104 so as to extend upwardly. And, the upper end portion of the voltage signal extraction portion 104v is connected to a terminal 105t of a board 105, which is provided with the voltage monitoring elements 101 and 102. Further, the terminal 105t of the board 105 and the voltage monitoring elements 101 and 102 are electrically connected to each other by a conductor 107 printed on the board 105.
FIG. 8 shows another electric tool battery 110, which is provided with a lead plate 114 connecting the negative electrodes of two cells S and the positive electrodes of two cells S. At the center of the lead plate 114, there is formed a voltage signal extraction portion 114v, which is electrically connected to a terminal (not shown) of a board 115, to which voltage monitoring elements (not shown) are mounted, by a signal wire 114r. 
In the above-described electric tool batteries 100 and 110, the lead plate 104, 114 connected to the electrodes of the cells S and the board 105, 115 provided with the two voltage monitoring elements 101 and 102 are connected by one voltage signal extraction portion 104v or the signal wire 114r. Thus, when, for example, poor connection is generated at the connection portion between the voltage signal extraction portion 104v and the board 105, it is not possible to monitor the cells S1 through Sn for overcharge or the like even if the two voltage monitoring elements 101 and 102 operate properly.
Thus, there is a need in the art to provide an electric tool battery in which even if poor connection or the like is generated in the wiring between one voltage monitoring element and the cells, it is possible to monitor the voltages of the cells by another voltage monitoring element, thereby achieving an improvement in terms of reliability against overcharge or the like of the electric tool battery.